Among rails for use in railways and the like, for example, the rail for use under a strict environment such as a stope of natural resources such as coals is required to have a high wear resistance and a high toughness. Such a rail has the high wear resistance, the high toughness and a high hardness because a structure of a rail head portion consists of a fine pearlite structure. The rail in which the structure of the head portion consists of the fine pearlite structure is usually manufactured by using the following manufacturing method.
First, a hot-rolled rail at a temperature that is not less than an austenite range temperature or a rail heated at the temperature that is not less than the austenite range temperature is conveyed into a heat treatment apparatus in an upright state. Here, the upright state refers to a state where the head portion of the rail is disposed upward and an underside of foot portion thereof is disposed downward. When the rail is conveyed into the heat treatment apparatus, for example, there is a case where the rail which remains in a rolled length of about 100 m is conveyed into the heat treatment apparatus or a case where the rail is cut into rails so that a length per rail is, for example, about 25 m (hereinafter also referred to as sawing), and then conveyed into the heat treatment apparatus. It is to be noted that in the case where the rail is sawn and then conveyed into the heat treatment apparatus, the heat treatment apparatus might be divided into zones each having a length corresponding to the sawn rail.
Next, in the heat treatment apparatus, toe tip portions of the rail are bound with clamps, and a rail head top portion, head side portions, the underside of foot portion and further a web portion as required are forcibly cooled by using a cooling medium. In the cooling medium, air, water, mist or the like is used. In such a manufacturing method of the rail, a cooling rate during the forcible cooling is controlled, whereby the whole head portion including an inner region of the rail can be formed into the fine pearlite structure. Furthermore, when forcibly cooling the rail, the cooling is performed until a temperature of the head portion of the rail reaches a range of about 350° C. to 450° C.
Furthermore, the rail bound with the clamps is released, and the rail is conveyed to a cooling bed. The rail conveyed to the cooling bed is cooled to about room temperature.
As for structure of the rail head portion, bainite is poor in wear resistance and martensite is poor in toughness. Therefore, it is difficult to simultaneously achieve a high wear resistance and a high toughness, and hence the whole head portion needs to have the pearlite structure. Furthermore, as lamella spacing of the pearlite structure is finer, both the wear resistance and the toughness improve, and hence the structure of the rail head portion needs to have the fine lamella spacing. For the purpose of obtaining the pearlite structure at the fine lamella spacing, it is important to set the cooling rate during the forcible cooling.
For example, in PTL 1, there is disclosed a method of manufacturing a pearlite-based rail containing, in terms of mass %, C: 0.65 to 1.2%, Si: 0.05 to 2.00%, and Mn: 0.05 to 2.00% and the remainder comprising Fe and inevitable impurities, and in the method, a rolling temperature and a head portion cumulative area reduction ratio are defined, and then accelerated cooling or natural radiation cooling of a rail head portion surface is performed down to at least 550° C. at a cooling rate of 2 to 30° C./sec.
Furthermore, in PTL 2, there is disclosed a method of rapidly cooling, down to 450 to 680° C. at a cooling rate of 2 to 20° C./sec, a rail head portion surface having a temperature of an A3 or Acm ray to 1000° C. in a hot-rolled rail containing, in terms of mass %, C: 0.60 to 1.20%, Si: 0.05 to 2.00%, and Mn: 0.05 to 2.00% and the remainder comprising Fe and inevitable impurities, afterward raising the temperature to a temperature range of the A3 or Acm ray to 950° C. at a temperature rise rate of 2 to 50° C./sec, afterward holding the temperature range for 1.0 to 900 sec, and further afterward performing accelerated cooling down to 450 to 650° C. at a cooling rate of 5 to 30° C./sec.
Furthermore, in PTLs 3 and 4, there is disclosed a method of performing cooling from an austenite range to a pearlite transformation temperature of generally about 600° C. at a cooling rate of 30° C./sec or less, holding a surface temperature until pearlite transformation almost finishes, and then performing cooling down to ordinary temperature range by use of a refrigerant as fast as possible.